Hollow filaments are known in the fiber market. These hollow fibers provide desirable properties, such as soil hiding, because of one or more continuous axially extending voids running through the filament. Hollow fibers may appear as bulked continuous filaments ("BCF") or staple (i.e., short length) fibers. BCF yarns are, however, becoming a standard of the synthetic fiber industry, due at least in part, to the improved performance and process efficiencies they represent.
Hollow fibers are known in various cross-sections, such as round or multilobal. Trilobal BCF filaments are known and are described in, for example, U.S. Pat. No. 5,208,107 to Yeh et al.
The invention described herein is a hollow fiber (preferably, but not essentially, trilobal BCF) yarn with an increased stable percent void space. "Percent void space" is the cross-sectional area occupied by the void.
When used for carpet applications, high void volume fibers permit carpet mills to use less fiber to produce desired carpet cover resulting in reduced manufacturing cost. Alternatively, the same amount (by weight) of fiber can be used to produce an increased cover product, i.e., an improved product manufactured without increasing the production cost. The size and number of the voids, as well as the cross-section of the filament, determine the properties of the filament, like soil-hiding, bulk, luster, etc. U.S. Pat. No. 5,208,107 to Yeh et al. describes certain hollow trilobal fibers. In order to obtain and maintain consistent, pre-determined properties, the characteristics of the voids should be as accurately specified and controlled as possible.
However, the size of the voids (relative to the cross-section of the fiber) is known to decrease during the manufacture of the filaments. The molten filaments emerge from the spinneret with voids of a target size, but once the filaments are quenched, the voids have shrunken. Also, for relatively large void spaces (greater than about 7%), obtaining void space closure is a problem associated with certain spinneret designs, especially those designs that rely on coalescence to achieve the hollow fiber cross-section, such as where three "y" shaped orifices are used to produce a single void hollow trilobal fiber. Various process parameters (polymer temperature, quench rate, polymer viscosity, etc.)can be adjusted to minimize the shrinkage of the void space and, to some degree improve the frequency of void space closure, but these adjustments can be made only by sacrificing the stability of the process. For example, increasing the quench rate by increasing the flow rate of the quench gas can cause the filaments to blow in the air, disturbing the process.
It is known to use additives to reduce void shrinkage. U.S. Pat. No. 5,318,738 to Agarwal et al. describes melt blending an N,N'-dialkyl polycarbonamide with molten fiber-forming polyamide prior to spinning into filaments. The N,N'-dialkyl polycarbonamide is a liquid at common ambient temperatures (e.g., around 25.degree. C.) and requires equipment capable of handling liquids. If such equipment is not already available at the manufacturing site, capital expenditure is required to use the Agarwal additive. It would be advantageous to have a normally solid material that does not require special liquid handling equipment.
It is also known that higher viscosity polymers generally have less void size shrinkage and less unclosed voids than similar polymers of relatively lower viscosity. Increased viscosity polymers are known to present spinning difficulties. Thus, the increase in polymer viscosity only improves void creation performance to a degree before problems are encountered with spinning performance.
A larger void size is desired but is not easy to manufacture because the open void formation during fiber manufacturing. An improved process has been found to overcome these deficiencies.